Steering-by-driving systems for vehicles are well known in the art. A wheeled vehicle having a steering-by-driving system includes at least two driven wheels located on opposite sides of the vehicle. Each driven wheel has an associated motor, typically hydraulic, operatively connected thereto. Each motor is independently drivable at a selected speed and direction. By controlling the speed and direction of each of the driven wheels, the movement and steering of the vehicle are controlled. When the wheels are driven in the same direction at the same speed, the vehicle moves in a straight path. When the wheels are driven in the same direction but one wheel is driven faster than the other, the vehicle turns in the direction of the slower driven wheel. When the wheels are driven at the same speed but in opposite directions, the vehicle turns about a vertical axis. Vehicles capable of having the wheels driven in opposite directions are said to have a zero turning radius.
Some known steering-by-driving systems for vehicles typically include separate levers for individually controlling each driven wheel. Speed, direction, and steering of such vehicles are controlled by the operator moving the levers forward and rearward. Other known steering-by-driving systems for vehicles include a single lever or "joystick" for speed, direction, and steering control. Still other known steering-by-driving systems for vehicles use toggle switches as the control mechanism. While the previous systems have provided speed and steering control of the vehicle, an operator, familiar with driving a road vehicle having a rotatable steering wheel, could find the prior known control systems to be less than satisfactory.
Also, previously known steering-by-driving systems for vehicles have had one steering ratio. Thus, these vehicles have a fixed relationship between the position of the control levers, for example, and the angular displacement of the vehicle, i.e., the vehicle steering angle. Because of this fixed relationship, known steering-by-driving systems have not been totally adequate for vehicles in which different steering ratios would be desirable, such as lawn mowers.
Furthermore, engines for driving hydraulic pumps in known hydraulic, steering-by-driving systems are subject to stalling. Engine stalling typically occurs when the vehicle driven wheels are subjected to a high load, e.g., the vehicle is traveling up a steep hill. When the driven wheels are loaded, the pump provides more fluid than the hydraulic motors of the driven wheels can utilize. When this occurs, the fluid pressure at the pump outlet increases which, in turn, results in the pump resisting the drive force of the engine thereby stalling the engine.
Also, when a vehicle having a steering-by-driving system goes down hill, the hydraulic motors draw more fluid than the pump can provide. When this occurs, the pump cavitates.